1. Field of the Invention
Embodiments of the invention generally relate to methods for processing substrates, and more particularly, to methods for oxide etching during plasma clean processes.
2. Description of the Related Art
In semiconductor, display, solar, and other electronics fabrication, a native oxide typically forms when a substrate surface is exposed to oxygen and water in the air. Oxygen exposure occurs when substrates are moved between processing chambers at atmospheric or ambient conditions, or when a small amount of oxygen remains in a processing chamber. Native oxides may also result from contamination during etching process. Native oxide films are usually very thin, such as between 5-20 Å, but thick enough to cause difficulties in subsequent fabrication processes. Therefore, a native oxide layer is typically undesirable and needs to be removed prior to a subsequent fabrication process.
Such difficulties usually affect the electrical properties of electronic devices formed on the substrate. For example, a particular problem arises when native silicon oxide films are formed on exposed silicon containing layers, especially during processing of metal oxide silicon field effect transistor (“MOSFET”) structures. Silicon oxide films are electrically insulating and are undesirable at interfaces with contact electrodes or interconnecting electrical pathways because they cause high electrical contact resistance. In MOSFET structures, the electrodes and interconnecting pathways include silicide layers formed by depositing a refractory metal on bare silicon and annealing the layer to produce the metal silicide layer. Native silicon oxide films at the interface between the substrate and the metal reduce the compositional uniformity of the silicide layer by impeding the diffusional chemical reaction that forms the metal silicide. This results in lower substrate yields and increased failure rates due to overheating at the electrical contacts. The native silicon oxide film can also prevent adhesion of other CVD or sputtered layers which are subsequently deposited on the substrate.
Sputter etch processes have been used to reduce contaminants in large features or in small features having aspect ratios smaller than about 4:1. However, sputter etch processes can damage delicate silicon layers by physical bombardment. In response, wet etch processes utilizing hydrofluoric acid have also been used to remove native oxides. However, wet clean etch processes are disadvantageous in smaller devices where the aspect ratio exceeds 4:1, and especially where the aspect ratio exceeds 10:1. Particularly, the aqueous solution has difficulty penetrating into vias, contacts, or other small features formed within the substrate surface. As a result, the removal of the native oxide film is incomplete. Similarly, a wet etch solution, if successful in penetrating a small feature, is even more difficult to remove from the feature once etching is complete. Also, wet etch processes usually have strict queue time control, create undesirable watermarks on the substrate, and have environmental concerns due to the large amount of hazardous liquid waste.
Another approach for eliminating native oxide films is a dry etch process, such as one utilizing fluorine (F2) gas. One disadvantage to using fluorine-containing gases, however, is that fluorine is typically left behind on the substrate surface. Fluorine atoms or fluorine radicals left behind on the substrate surface can be detrimental. For example, the fluorine atoms left behind can continue to etch the substrate causing voids therein.
A more recent approach to remove native oxide films has been to form a fluorine/silicon-containing salt on the substrate surface that is subsequently removed by a thermal annealing process. In this approach, a thin layer of the salt is formed by reacting a fluorine-containing gas with the silicon oxide surface. The salt is then heated to an elevated temperature sufficient to decompose the salt into volatile by-products which are then removed from the processing chamber. The formation of a reactive fluorine-containing gas is usually assisted by thermal addition or by plasma energy. The salt is usually formed at a reduced temperature that requires cooling of the substrate surface. This sequence of cooling followed by heating is accomplished by transferring the substrate from a cooling chamber where the substrate is cooled to a separate anneal chamber or furnace where the substrate is heated.
For various reasons, this reactive fluorine processing sequence is not desirable. Namely, throughput is greatly diminished because of the time involved to transfer the substrate. Also, the substrate is highly susceptible to further oxidation or other contamination during the transfer between chambers. Moreover, the cost of ownership is doubled because two separate chambers are needed to complete the oxide removal process.
Therefore, there is a need for a method to remove or etch native oxides while passivating the underlying substrate surface, preferably, within a single processing chamber.